1. Field of the Invention
The present invention relates to a display control device and, more particularly, to a display control device for a portable computer which is adaptable to both a color CRT display and a flat-panel display.
2. Description of the Related Art
Various laptop or notebook types of portable computers have been developed recently.
Each of such portable computers is equipped with a flat-panel display such as a plasma display or a liquid-crystal display. The flat-panel display is mounted on a computer body so that it can be rotated between its closed position and its open position. The flat-panel display is designed to, in its closed position, cover a keyboard formed integral with the computer body. Thus, the computer is easy to carry. For this reason, the flat-panel display is suitable for portable computers in improving their portability. Most of flat-panel displays used with portable computers are displays for monochrome gradation display. The monochrome gradation flat-panel displays represent colors in black, white, and shades of gray, and thus are very inexpensive as compared with color flat-panel displays.
Recent portable computers, on which monochrome or color flat-panel displays are standard, are each quipped with video output terminals so that they can be connected with a color CRT display as required. Color CRT displays have been widely used as monitors for desktop personal computers. For this reason, a large number of application programs have been developed for color CRT displays. Therefore, a portable computer that can use a color CRT display permits effective utilization of existing software resources and thus can make the same practical applications as a desktop personal computer.
A flat-panel display and a color CRT display require their respective individual display control. In the case of a monochrome flat-panel display in particular, it is required to convert display data with color information for color CRT displays to monochrome gradation video data.
For this reason, a display control device for use in a portable computer equipped with a monochrome flat-panel display is equipped with a RAMDAC (color-table-equipped digital converter) for converting display data with color information to color video signals for a color CRT display and a flat-palette table for converting the display data to monochrome gradation video data for the monochrome flat-panel display.
The RAMDAC is arranged to convert display data with color information to R, G, and B analog color video signals and has,, for example, 256 color registers. In each of the color registers, R, G, and B color data are stored. The flat-palette table is adapted to convert display data with color information to monochrome gradation video data and has as many gradation registers as there are color registers in the RAMDAC color table, i.e., 256 gradation registers. In each of the gradation registers gradation data indicating a gradation (black, white, or a shade of gray) is stored.
In making a color display on a color CRT display, color-information-carrying display data of, for example, eight bits per pixel is applied to the RAMDAC as an index, whereby one of 256 color registers in the RAMDAC is selected. R, G, and B color data stored in the selected color register are converted to R, G, and B analog color video signals by respective individual digital-to-analog converters for application to the color CRT display. In making a monochrome display on a flat-panel display, on the other hand, the flat-palette table is referred to by the same display data of eight bits per pixel as is used for selecting among color registers in the RAMDAC, so that one of the 256 gradation registers is selected. The gradation data stored in the selected gradation register is applied to the flat-panel display as monochrome gradation video data.
As described above, the prior art display control device converts display data having color information originally intended for a color CRT display to gradation video data through the flat-palette table, thereby realizing monochrome gradation display on a flat-panel display. In this case, tones of monochrome gradation display on the flat-panel display are determined by values of gradation data set in the gradation registers in the flat-panel table. In order to reproduce tones of a color display on a color CRT display with a monochrome gradation display on a flat-panel display, therefore, it is necessary to generate monochrome gradation data associated with color data set in the RAMDAC and set them in the flat-panel table.
In general, the number of bits of a piece of gradation data to be set in the flat-pellet table is smaller than the number of bits of a piece of color data to be set in the RAMDAC. The reason is that the flat-panel display is small in the number of gradations per dot that can be reproduced because of panel's characteristics, and, for example, up to 16 gradations can be reproduced. For this reason, the prior art display control device uses a technique of simply thinning bits of each piece of color data set in the RAMDAC for linear conversion from color data to gradation data.
However, in the process of converting color data to gradation data, no consideration is given to the human visual sense of colors in which the sensitivity to lightness increases in the order of blue, red, and green. Thus, if gradation data prepared by using the above-mentioned linear conversion technique are used, then tones of a monochrome display on the flat-panel display will look different from those of a color display on the color CRT display. That is, the monochrome flat-panel display cannot faithfully reproduce tones of color display that can be displayed on a color CRT display.
In the prior art, monochrome gradation data is always generated utilizing color data in the RAMDAC as described above. Thus, it is impossible to generate any gradation data that is not associated with color data in the RAMDAC and display it on a flat-panel display. For example, depending upon application programs to be executed, if a color display image, as a whole, assumes, for example, greenish color, a monochrome display image may become difficult to visually recognize. For, color data in the RAMDAC are originally prepared for color CRT displays that can reproduce a large number of colors, and hence it is difficult to reproduce delicate changes in color on a flat-panel display having a limited number of reproducable gradations.
Moreover, in the prior art, in order to support monochrome gradation display on a flat-panel display, it is required to install a flat-palette table with as many as 256 gradation registers. This will result in complication of the hardware configuration of the display control device.
Furthermore, even color flat-panel displays, having a limited number of reproducable gradations per dot because of panel's physical characteristics, involve difficulties in faithfully reproducing tones of color display on color CRT displays and displaying as many colors as color CRT displays can.
Difficulties with the prior art display control device lie in faithfully reproducing the tones of color display of CRT displays on a flat-panel display and realizing on the flat-panel display any monochrome gradation display that is not associated with color data for CRT displays because monochrome gradation data are always produced from the color data.